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10a
for we say explicitly in perek Tevul Yom UMechusar Kippurim {Zevachim perek 12, daf 100b} (Vayikra 10:19, Aharon talking about not eating a korban on the day Nadav and Avihu died)
"for me {Aharon}, the day is forbidden and the night is allowed. However, for {ensuing} generations, whether day or night is forbidden. These are the words of Rabbi Yehuda. Rabbi Shimon {or perhaps just Rabbi} says the aninut of night is not Biblical {midivrei torah} but rather midivrei sofrim {Rabbinic}.and we also say there {immediately preceding the aforementioned brayta}:
The Sages said this before Rava, andUntil when are we mitanenim {acting with aninut} upon him? That entire day, without its night. Rabbi says: the entire time that he is not buried {and our girsa there is: and if he is buried (the day)} grabs its night.
he said: from the fact that Rabbi says that the day of burial grabs its night with it Rabbinically, we deduce that the day of death grabs its night Biblically, and Rabbi holds that aninut of the night is Biblical.and we also learn in Pesachim {daf 91b} that an onen immerses himself and eats his {korban} pesach at evening, which shows that he maintains that aninut at night is only Rabbinic and as regards the {korban} pesach, they did not establish their words in a situation of karet {for avoiding eating the pesach}. And since we find all these Tannaim who hold that only the first day is Biblical, but its night which is the second night to the day of death it only grabs Rabbinically, and it is only Rabbi Yehudah who maintains that the day of death grabs its night Biblically, we derive that the halacha is not like him, for it is established for us, the indivudual vs. the many, the halacha is like the many.
But we learnt {in a brayta}:Really I will tell you that aninut of night is not Biblical but rather Rabbinic, and the Sages strengthened their words more than those of the Torah.
(Vayikra 10:19)"for me {Aharon}, the day is forbidden and the night is allowed. However, for {ensuing} generations, whether day or night is forbidden. These are the words of Rabbi Yehuda. Rabbi {and it does not say Shimon} says the aninut of night is not Biblical {midivrei torah} but rather midivrei sofrim {Rabbinic}.
Therefore, the first day, which is the day of death, it is Biblical, but the night of the second day and on is Rabbinic, and this is the opinion of Rabban Gamliel, who holds that aninut of the night is Rabbinic, and the halacha is like him.
And these seven days {of mourning}, the Sages did not base it on Bereishit 50:10:
י וַיָּבֹאוּ עַד-גֹּרֶן הָאָטָד, אֲשֶׁר בְּעֵבֶר הַיַּרְדֵּן, וַיִּסְפְּדוּ-שָׁם, מִסְפֵּד גָּדוֹל וְכָבֵד מְאֹד; וַיַּעַשׂ לְאָבִיו אֵבֶל, שִׁבְעַת יָמִים. | 10 And they came to the threshing-floor of Atad, which is beyond the Jordan, and there they wailed with a very great and sore wailing; and he made a mourning for his father seven days. |
י וְהָפַכְתִּי חַגֵּיכֶם לְאֵבֶל, וְכָל-שִׁירֵיכֶם לְקִינָה, וְהַעֲלֵיתִי עַל-כָּל-מָתְנַיִם שָׂק, וְעַל-כָּל-רֹאשׁ קָרְחָה; וְשַׂמְתִּיהָ כְּאֵבֶל יָחִיד, וְאַחֲרִיתָהּ כְּיוֹם מָר. | 10 And I will turn your feasts [your chags] into mourning, and all your songs into lamentation; and I will bring up sackcloth upon all loins, and baldness upon every head; and I will make it as the mourning for an only son, and the end thereof as a bitter day. {P} |
Just like your chag {=Succot festival} is seven, so too mourning is seven.
And if you ask from what we learn in masechet moed katan, in perek eilu megalchin {perek3, daf 14b}: A mourner does not conduct himself in mourning during the festival, for it is written (in Dvarim 16:14)